Page 261 - Fiber Optic Communications Fund

242 Fiber Optic Communications

where is the fiber-loss coefficient and L is the transmission distance. From Eq. (5.161), we have
RA A
T LO −L∕2
I = e s
x x
2
RA A
r LO
= s x
2
RP 0
= s x
2
0.9 × 1.259
= 1∠∕4mA = 0.566∠∕4,
2
I = Re (I )= 0.4006 mA,
I,x x
I =−Im (I )=−0.4006 mA.
Q,x x
Similarly,
RA A
I = T LO −L∕2 s y
e
y
2
RA A
r LO
= s y
2
RP 0
= s y
2
0.9 × 1.259
= 1∠5∕4mA = 0.566∠5∕4,
2
I I,y = Re (I )=−0.4006 mA,
y
I =−Im (I )= 0.4006 mA.
Q,y y

Exercises

5.1 Compare qualitatively the features of a direct detection receiver with a coherent detection one.
5.2 Discuss the strengths and weaknesses of pn, pin, and Schottky barrier photodiodes. Which one would
you choose for a 10-Gb/s fiber-optic receiver? State the reasons for your choice of structure.
5.3 Compare the characteristics of avalanche photodiodes and pin photodiodes. Which one would you
choose for a 10-Gb/s fiber-optic receiver? If the bit rate is increased to more than 40 Gb/s, would your
choice remain the same or would you switch to the other photodiode. Why?

5.4 Avalanche photodiodes can be made as in the following structures: normal pn junctions, SAM, SACM,
and SAGCM. Compare and contrast the characteristics of each structure. Which structure is most
suitable for a low-Gb/s fiber-optic receiver?
5.5 Discuss the key design features of a RCE photodiode.